Low-drop voltage regulator

ABSTRACT

A follower amplifier which connects a variable and noisy power source to a load obtains its reference from a divider composed of two serially connected transistors using feedback to obtain two widely different ratios of dynamic resistance to average resistance.

C Umted States Patent [151 3,641,423

Stauffer 1 Feb. 8, 1972 [54] LOW-DROP VOLTAGE REGULATOR 3,513,378 5/1970 Kemper ..323/9 3,246,233 4/1966 Herz ......323/9 X [72] Invent Reuben Laverne 3,391,330 7/1968 Grossoehme ..323/22 T [73] Assignee: The Bendix Corporation Primary ExaminerA. D. Pellinen 22 Fld. N .161970 1 l e Attorney-Flame, Hartz, Smith & Thompson, Bruce L. Lamb [21] Appl. No.2 89,726 and William G. Christoforo 52] us. ca ..323/9, 323/19, 323/38 [571 ABSTRACT [51] Int. Cl. ..Gf 3/14 A follower amplifier which connects a variable and noisy [58] Field of Search ..323/4, 9, 16, 19,22 T, 38 power Source to a load obtains its reference from a divider composed of two serially connected transistors using feedback [56] References cued to obtain two widely different ratios of dynamic resistance UNITED STATES PATENTS average 3,305,725 2/1967 Huge et a1 ..323/9 X 9 Claims, 3 Drawing Figures airs F Pmuramw a ma 3.641 #123 FIG. 1

216 33 r FIG. 2

IO :1 l3 22 FIG. 3

, iNVENTOR REUBEN L.STAUFFER ATTORNEY LOW-DROP VOLTAGE REGULATOR BACKGROUND OF THE INVENTION This invention relates to DC regulated power supplies and more particularly to such power supplies which provide a noise-free voltage level at their output which is close to their input voltage level.

A common form of an open loop voltage regulator uses a junction transistor operating as an emitter-follower amplifier to refer a load to a zener diode voltage reference. When this type of voltage regulator is used to supply power to an automobile radio receiver and where the regulator input voltage varies from 10.5 to 16 volts and includes noise as could be expected from an automobile electrical system, the regulator being required to provide 8.5 volts across the load, this regulator is deficient in its ability to regulate the output voltage and attenuate noise from the power source. Additionally, the thermal inertia of the transistor is so low that it is almost instantly destroyed by a short circuit and thus cannot be protected by a fuze, even if the voltage drop across a fuze could be tolerated.

SUMMARY OF THE INVENTION A new voltage regulator is described herein wherein a current source having a high dynamic resistance supplies power to an improved zener diode circuit which provides the reference for the emitter-follower. The current source is based by the potential drop across a device having a low dynamic resistance connected in series with a high-resistance device as a divider across the power supply. Additionally, a low-resistance sensing means is provided for sensing load current and providing a feedback to attenuate emitter-follower drive should the load current rise above acceptable limits.

The circuits to be described may be used to provide a low noise stabilized potential directly to a useful load or may be combined with an emitter-follower to serve larger loads. Where the emitter-follower is used the voltage regulator per mits the use of a simple feedback circuit to limit the current that can be drawn through the emitter-follower to a safe value.

It is thus an object of this invention to provide a low-loss voltage regulator.

It is another object of this invention to provide a voltage regulator having high noise immunity.

It is still another object of this invention to provide a DC voltage regulator which only slightly attenuates the available voltage source but which greatly attenuates power supply voltage fluctuations, especially those rapid changes observable as noise.

It is still one more object of this invention to provide a DC voltage regulator of the type described which also includes means for limiting load current to a safe value.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic of one embodiment of the invention.

FIG. 2 is a partial schematic showing an improved version of the invention.

FIG. 3 is another partial schematic showing additional improvements to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the following description identical numerals refer to identical elements in the various figures. Referring now to FIG. 1, there is seen generally a voltage regulator receiving an input voltage from a battery 10, such as an automobile battery in an automobile electrical system, the regulator supplying a regulated noise-free voltage to a load schematically shown as resistor 35. As embodied here, the invention might typically be used to supply power to an automobile radio load 35 from the automobile battery 10. In this application it can be expected that the potential from battery will vary from a minimum of 10.5 volts to a maximum of 16.0 volts while the voltage supplied across load 35 must remain constant at 8.5 volts under a load of I50 milliamperes of current. Additionally, there might be expected impressed across battery 10 and superimposed thereon a noise signal in the order of 1.5 volts peak to peak. Connected across battery 10 from its positive to its negative terminal is the serially connected circuit of resistor 11, diode 13 and resistor 15, respectively. A PNP- transistor 17 has its base electrode connected to the junction between resistor 11 and diode 13, its emitter electrode connected to positive terminal of battery 10 and its collector electrode connected to the junction between diode l3 and resistor 15 and also to the base electrode of a transistor 22.

PNP-transistor 22 has its emitter electrode connected through resistor 20 to positive terminal of battery 10 and its collector electrode connected to regulated voltage line 26, which in turn i connected to the base electrode of NPN- transistor 33. A zener diode 25 has its cathode connected to line 26 and its anode connected through resistor 27 to the negative terminal of battery 10. NPN-transistors 23 and 28 have commonly connected collector electrodes connected to line 26 and commonly connected emitter electrodes connected to the negative terminal of battery 10. The base electrode of transistor 23 is connected to the junction between zener diode 25 and resistor 27. Previously mentioned transistor 33 has its collector electrode connected through resistor 31 to the positive side of the battery and its emitter electrode connected through load resistor 35 to the return of the battery. Another PNP-transistor 30 has its base electrode connected to the collector of transistor 33, its emitter electrode directly connected to the positive side of the battery and its collector terminal connected to the base electrode of transistor 28.

The high impedance at the collector of transistor 22 is further increased, in known fashion, by the connection of its emitter to the positive side of the battery through resistor 20 to provide current-dependent feedback.

The base electrode of transistor 22 is connected to the positive terminal of battery 10 through the serial arrangement of the emitter-base circuit of transistor 17 with diode 13. This provides a bias potential fixed with respect to the positive side of the battery at the transistor 22 base electrode. The junction potential of diode 13 is further stabilized by resistor 11 connected across the base-emitter junction of transistor 17. The current through this resistor also flows through the diode and is proportional to the nearly constant potential across the base-emitter junction of transistor 17. In this manner the current through diode 13 is made to be relatively independent of the current gain of transistor 17 with the result that the diode dynamic resistance is greatly reduced.

By virtue of the operation of transistor 17 as a current amplifier a much larger current will flow between its collector and emitter than between its base and emitter. The bias voltage applied to the base of transistor 22 will then be determined, as aforementioned, by the sum of the nearly constant tumon potential across the base-emitter junction of transistor 17 and the junction potential of diode 13. These aforementioned currents flow through resistor 15 together with the base current of transistor 22. It will be noted that the potential drop between the positive terminal of battery 10 and the base electrode of transistor 22 as taken through resistor 20 and the emitter-base junction of transistor 22 must be identical to the potential drop resulting from the sum of the potential drops of the emitter-base circuit of transistor 17 and the junction potential of diode 13. Thus, the potential drop across resistor 20 is very nearly equal to the potential drop across diode 13. As a result of the low potential drop between the emitter and collector of transistor 22 the resultant regulated voltage on line 26 can be depressed with respect to the voltage at the positive terminal of battery 10 by an amount as small as approximately the potential drop across a single diode.

A constant voltage circuit comprised of transistor 23, together with zener diode 25 and resistor 27 replace a single zener diode commonly found in the prior art. Zener diode 25 connected between the collector and base electrodes of transistor 23 provides a low dynamic resistance feedback circuit between these electrodes which results in a very low dynamic impedance between the collector and emitter electrodes transistor 23. The current flow between the regulated line 26 and the negative side of battery 10, assuming transistor 28 to be nonconductive, will divide so that the collector current of transistor 23 will equal the product of its base current and the current gain of that transistor. Resistor 27, connected between the base and emitter of transistor 23, is used to provide a substantially fixed additional current to zener diode 25, generally larger than the transistor 23 base current flowing through the zener, to make the total current gain of transistor 23 so that the feedback'circuit through the zener is maintained even under conditions requiring small base currents in transistor 23. This, combined with the fact that transistor 22 operates as a resistor, provides a much higher dynamic impedance between regulated 26 and the positive terminal of battery 10. This combination of dynamic impedances provides excellentnoise isolation at regulated line 26 in addition to providing, as previously discussed, a possible very low voltage drop between the regulated line and battery 10.

A useful load requiring only a few milliamperes of current can now be connected between regulated line 26 and the negative side of battery 10, where it will be found that the noise derived from the power supply has been reduced by a factor of 0.0003 as measured on line 26. Stable and noise-free potential of regulated line 26 can be more frequently used to bias the base electrode of an emitter follower, such as transistor 33, to drive a load 35. In this case, essentially the entire load current also flows through resistor 31. This resistor is quite small so that the normal voltage drop thereacross is not sufficient to forward bias transistor 30. However, if abnormally large load currents are drawn, such as by the short circuiting of the load, the voltage drop across resistor 31 will be sufficient to forward bias transistor 30, its collector current now flowing into the base of transistor 28 to render the latter transistor conductive to reduce the potential on regulated line 26..In this manner transistor 33 will be protected from overload and thermal runaway.

Referring now to FIG. 2, there is seen the schematic of a simplified version of the device of FIG. 1 having certain advantages and disadvantages over that circuit. Transistor 28 of FIG. 1 has been eliminated and the collector electrode of transistor 30 has here been connected directly to the base electrode of transistor 23 at its common terminal between zener diode 25 and resistor 27. In this case, and as a result of excess load current flowing in load 35 the voltage drop across resistor 31, as before, forward biases transistor 30 so that now its collector current divides between resistor 27 and the baseemitter circuit of transistor 23. Although it might appear that one of the effects of this arrangement would be to raise the voltage at the anode of diode 25 to thus raise the voltage of regulated line 26 and thereby aggravate the overload condition, in actuality, the initial reaction of this circuit is to increase current flow through transistor 23 to thereby depress the voltage on line 26 to protect the circuit from thermal runaway. The reaction of this circuit to overload is more rapid than the reaction of the circuit of FIG. 1 since transistor 23 is already somewhat conductive during normal operation of the circuit. This, in addition to the fact that a transistor may be eliminated, comprise the advantages of this circuit. Some disadvantage is present however, in the fact that transistor 30 collector leakage currents will slightly upset the operation of the zener diode, causing some undesirable fluctuation of voltage on controlled line 26.

A field effect transistor with its gate connected to a source electrode is commercially available and has been designated as a current-regulating or current-limiting diode. With several volts applied between its terminals it exhibits a high dynamic resistance. It may be substituted advantageously for resistor of FIG. 1. Its use will also permit a modification of the left side of FIG. 1 as illustrated in FIG. 3, where the cathode of the current-limiting diode 41 is connected to the negative side of battery 10 and its anode is connected to the base of transistor 22 in common with the cathode of diode 13. The anode of the latter diode is connected to the positive terminal of battery 10 through another diode 40, rather than through the baseemitter junction of transistor 17 of FIG. 1. For this reason,

transistor 17 and resistor 11 of FIG. 1 are omitted. The current-limiting diode must pass current somewhat in excess of that required for the base of transistor 22 under the most adverse combination of tolerances.

Diodes l3 and 40 are generally selected to be of the same type, and selected to provide a low dynamic resistance to the range of currents that may flow through them. The base current of transistor 22 is predictable only within a range of eight to one and there is a manufacturing tolerance on the currentlimiting diode 41, thus the range of current through diodes l3 and 40 is substantial. To keep the ratio of maximum to minimum current through these diodes within reasonable bounds, the least value of the current range for the current limiting diode is designed to significantly exceed the greatest value of the current range of current at the base of the transistor 22.

The invention has been described as it would appear if all the semiconductor devices were made of the same base materials. Some obvious changes would be made in the circuit if more than one kind of material appeared in the circuit. For example, if transistor 22 of FIG. 3 were germanium and one of the diodes 13 and 40 were silicon, the other diode might be omitted or the value of resistor 20 increased to obtain the same results. Or if a field effect transistor were substituted for one of the junction transistors, consideration of the base current would vanish. Also, the polarity of the power source may be reversed with respect to the circuit if semiconductor devices of opposite polarity are substituted for those shown. These and other modifications of the invention might be made by one skilled in the art without departing from the true spirit and scope of the invention. Accordingly, the invention is to be limited only in accordance with the appended claims.

The invention claimed is:

1. In a voltage regulator receiving input power from a power supply having first and second terminals, an output terminal, a first current-limiting circuit having a ratio of dynamic resistance to average resistance large compared to unity connected between said first and output terminals, and a potential-limiting circuit having a ratio of dynamic resistance to average resistance small compared to unity connected between said output terminal and said second terminal, an improvement wherein said first current-limiting circuit comprises:

a resistor;

a first transistor having an emitter electrode connected through said resistor to said first terminal, a collector electrode connected to said output terminal, and a control electrode;

means providing a first constant potential drop connected between said first terminal and said first transistor control electrode; and wherein said potential-limiting circuit comprises:

a second transistor having an emitter electrode connected to said second terminal, a collector electrode connected to said output terminal, and a control electrode; and

means providing a second constant potential drop connected between said output terminal and said second transistor control electrode.

2. The improvement recited in claim 1 wherein said means providing said first and second constant potential drops are semiconductor diodes.

3. The improvement recited in claim 2 with an additional resistor connected between said second transistor emitter and control electrodes.

4. The improvement recited in claim 1 wherein said means for providing a first constant potential drop comprises:

a second current-limiting means;

a third transistor having an emitter electrode connected to said first terminal, a collector electrode connected to the control electrode of said first transistor, and a control electrode; and,

means providing a constant potential difference connected between said third transistor control and collector electrodes.

5. The improvement recited in claim 4 with an additional resistor connected between said third transistor emitter and control electrodes.

6. The improvement as recited in claim 1 with additionally an emitter follower transistor having a control electrode connected to said output terminal, an emitter electrode connected to said output terminal, an emitter electrode adapted for having a load connected therefrom to said second terminal, and a collector electrode; and,

means for electrically connecting said emitter follower collector electrode to said first terminal.

7. The improvement recited in claim 6 wherein said means for electrically connecting comprises means for sensing current in said emitter follower collector emitter circuit and with additionally means responsive to said sensed current for reducing voltage at said output terminal.

8. The improvement recited in claim 6 wherein said means for electrically connecting is a resistor connected between said first terminal and said emitter follower collector electrode and with additionally:

a further transistor having an emitter electrode connected to said first terminal, a control electrode connected to said emitter follower collector electrode and a collector electrode connected to said second transistor control electrode.

9. The improvement recited in claim 6 wherein said means for electrically connecting is a resistor connected between said first terminal and said emitter follower collector electrode and with additionally:

a further transistor having an emitter electrode connected to said first terminal, a control electrode connected to said emitter follower collector electrode and a collector electrode; and,

a second further transistor having an emitter electrode connected to said second terminal, a collector electrode connected to said output terminal and a control electrode connected to said further transistor collector electrode. 

1. In a voltage regulator receiving input power from a power supply having first and second terminals, an output terminal, a first current-limiting circuit having a ratio of dynamic resistance to average resistance large compared to unity connected between said first and output terminals, and a potential-limiting circuit having a ratio of dynamic resistance to average resIstance small compared to unity connected between said output terminal and said second terminal, an improvement wherein said first current-limiting circuit comprises: a resistor; a first transistor having an emitter electrode connected through said resistor to said first terminal, a collector electrode connected to said output terminal, and a control electrode; means providing a first constant potential drop connected between said first terminal and said first transistor control electrode; and wherein said potential-limiting circuit comprises: a second transistor having an emitter electrode connected to said second terminal, a collector electrode connected to said output terminal, and a control electrode; and, means providing a second constant potential drop connected between said output terminal and said second transistor control electrode.
 2. The improvement recited in claim 1 wherein said means providing said first and second constant potential drops are semiconductor diodes.
 3. The improvement recited in claim 2 with an additional resistor connected between said second transistor emitter and control electrodes.
 4. The improvement recited in claim 1 wherein said means for providing a first constant potential drop comprises: a second current-limiting means; a third transistor having an emitter electrode connected to said first terminal, a collector electrode connected to the control electrode of said first transistor, and a control electrode; and, means providing a constant potential difference connected between said third transistor control and collector electrodes.
 5. The improvement recited in claim 4 with an additional resistor connected between said third transistor emitter and control electrodes.
 6. The improvement as recited in claim 1 with additionally an emitter follower transistor having a control electrode connected to said output terminal, an emitter electrode adapted for having a load connected therefrom to said second terminal, and a collector electrode; and, means for electrically connecting said emitter follower collector electrode to said first terminal.
 7. The improvement recited in claim 6 wherein said means for electrically connecting comprises means for sensing current in said emitter follower collector emitter circuit and with additionally means responsive to said sensed current for reducing voltage at said output terminal.
 8. The improvement recited in claim 6 wherein said means for electrically connecting is a resistor connected between said first terminal and said emitter follower collector electrode and with additionally: a further transistor having an emitter electrode connected to said first terminal, a control electrode connected to said emitter follower collector electrode and a collector electrode connected to said second transistor control electrode.
 9. The improvement recited in claim 6 wherein said means for electrically connecting is a resistor connected between said first terminal and said emitter follower collector electrode and with additionally: a further transistor having an emitter electrode connected to said first terminal, a control electrode connected to said emitter follower collector electrode and a collector electrode; and, a second further transistor having an emitter electrode connected to said second terminal, a collector electrode connected to said output terminal and a control electrode connected to said further transistor collector electrode. 